In an optical transmission system, transmission characteristics of the transmission system are measured at the time of installation and a discrimination threshold of a received signal light at a receiving terminal is determined according to the obtained result. Thereafter, a signal value of the received signal light is discriminated with the discrimination threshold.
As mentioned above, in conventional systems, the discrimination thresholds are fixed. However, it has been understood that the transmission characteristics of the optical transmission line fluctuate with time and, consequently, the optimum discrimination threshold of the received signal light varies as well. FIG. 18 shows a measured result of a time variation of an optimum discrimination threshold. The vertical line and horizontal line show the optimum discrimination threshold and elapsed time respectively.
If the discrimination threshold of the received signal light is remained at the fixed value in spite of such variation, a bit error rate (Q value) becomes deteriorated.